Waveform distortions frequently occur in the signal transmission path of a composite television signal from a television signal transmitting station to a receiver. The transmission path includes the internal circuits of the television receiver receiving such a signal. These distortions are in the form of both amplitude and phase distortions and are observed in the reproduced picture at the receiver in the form of smearing, ringing, ghosts, etc. The distortion is highly undesirable since it directly affects the quality of the reproduced picture on the viewing screen of the television receiver, which, in extreme cases, can make viewing of the reproduced image an unpleasant experience.
The resultant distortion is produced by a variety of causes, which may include any one or all of: reflection of the transmitted signals by buildings, mountains, or other obstructions; impedence mismatching between the receiving antenna or cable and the television receiver; and any deviations within the receiver itself of the filters in the intermediate frequency amplifier circuit or the time delay circuit in the video section of the receiver. Any deviation in the characteristics of any of these potential sources of distortion from the ideal is likely to result in a degradation of the reproduced image in the television receiver. The degree of degradation of the image varies in accordance with the degree of distortion caused by any one or more of these possible sources of distortion.
It has been found that, in most cases, the distortion from any of the causes mentioned above is linear distortion rather than non-linear distortion. Consequently, it is possible to compensate for the waveform distortion by inserting into the television signal transmission path a filter which has a frequency characteristic which is a mirror image of or opposite to the characteristic which created the waveform distortion in the first place. In the past, systems for achieving this purpose have included a transversal filter and a reference signal generator controlled by a training signal located in the composite video signal for synchronizing the operation of the reference signal generator and the operation of the transversal filter. The reference signal then is compared with the received training signal and the result of the comparison is used to control the tap coefficients of the transversal filter for automatically compensating for the waveform distortion. A system for accomplishing automatic equalization in this manner is disclosed in the patent to Shimano and Aizawa, U.S. Pat. No. 4,044,381, issued Aug. 23, 1977. In the Shimano et al. patent, the training signal which is utilized to control the operation of the reference signal generator is obtained from the horizontal or vertical synchronizing signal or from an additional signal added to the video signal as a reference. The active video signal itself cannot be used as a reference because it changes continuously.
A problem which exists in using a portion of either the horizontal or vertical synchronizing signals is that, as is well known, such signals at this level are subject to many distortions which are not experienced by the active video signals. As a consequence, a system of the type disclosed in the Shimano et al patent will correct for distorted synchronization signals, but at the same time may actually distort the video. It is the video signal which is observed on the face of the picture tube and not the synchronization signals. Consequently, proposals have been made of inserting a special signal for utilization in waveform distortion correction, and one such signal also is disclosed in the Shimano et al. patent. The problem with utilizing any additional signal added to the already very complex composite television signal is that such new training signals usually are time division multiplexed into the vertical interval of the composite video. It is possible that these additional signals may result in visible interference on the screen of the television receivers. Even more significant, however, is that the space in the vertical interval where such additional signals may be placed is badly needed for proposed applications such as teletext data, bilingual sound, and the like. These latter potential uses of the limited available space have a higher priority than an additional signal usable only for equalization.
A system for correcting for linear distortion also is distinguishable from a system for removing ghost signals from received video signals. It has been shown that linear distortions can be considered to result from paired echoes which are closely associated with the main or desired response. When the paired echoes become widely removed from the main or desired response, they generally are considered as ghosts and appear as a spaced part second image on the television screen. Although linear distortion is in the form of paired echoes, television ghosts normally do not occur in pairs but instead appear in the form of multiple ghosts (more than pairs). Consequently, any solution to removing ghosts from a video signal requires a considerably more complex system and, particularly, requires a very long (high number of sections) transversal filter, or the like. A system using a transversal filter for ghost elimination in a television receiver is disclosed in the patent to Iwasawa et al., U.S. Pat. No. 4,127,874, issued Nov. 28, 1978. This patent is mentioned merely as typical of a ghost elimination system for a television receiver.
It is desirable to provide for a simplified automatic waveform equalization system for correcting for linear distortion in a television receiver which is capable of using signals already present in the composite television signal but which provides correction more closely related to the distortion of the video portion of the signal than the synchronizing signal portion. Furthermore, it is desirable to combine the functions of the linear distortion correction with other functions already present in the television receiver to reduce the overall cost of providing distortion correction or equalization.